1. Field of the Invention
The present invention relates to methods and systems for remediating contaminated water and soil, and, more particularly, to such methods and systems for decontaminating sites from organics and heavy metals.
2. Description of Related Art
Following a long period of environmental neglect, the United States and other countries have placed a high priority on remediating contaminated sites. It is estimated that between 300,000 and 400,000 contaminated sites are scheduled for cleanup in the United States in the coming decades, at an estimated cost as high as $500 billion to $1 trillion (National Research Council, xe2x80x9cAlternatives for Ground Water Cleanup, Washington, D.C.: National Academy Press, 1994; M. Russell et al., Hazardous Waste Remediation: The Task Ahead, Knoxville: University of Tennessee, 1991). U.S. spending on waste site remediation totaled approximately $9 billion in 1996 alone.
Despite this considerable investment, conventional technologies for remediating contaminated sites, especially those with contaminated ground water, are inadequate. For example, the National Research Council has conducted a study of conventional ground water cleanup systems at 77 contaminated sites and determined that ground water cleanup goals had been achieved at only 8 of the sites and that full achievement was highly unlikely with the in-place technologies at 34 of the 77 sites (NRC, ibid., 1994; MacDonald and Kavanaugh, Envir. Sci. Tech. 28(8), 362A-68A, 1994). Based on these findings, it is believed that improved technologies are needed to restore contaminated sites.
The most common types of contaminants found at waste sites include chlorinated solvents, petroleum hydrocarbons, and metals (NRC, 1994). Chlorinated solvents, such as trichloroethane (TCE) and perchloroethylene (PCE), are used for such purposes as dry cleaning and degreasing industrial manufacturing equipment and cleaning military aircraft. Petroleum hydrocarbons commonly found in ground water include the components of gasoline, such as benzene, toluene, ethylbenzene, and xylene. Other common contaminants of ground water include naphthalene and chlorinated solvents. Because of the widespread use of both chlorinated solvents and petroleum hydrocarbons, contaminated ground water has been found in many sites around the world. Additional ground-water and soil contaminants comprise polycyclic aromatic hydrocarbons (PAHs), created from combustion, coal coking, and process, petroleum refining, and wood-treating operations; and polycholorinated biphenyls (PCBs), once widely used in electrical transformers and capacitors and for a variety of other industrial purposes.
Some conventional technologies for cleaning contaminated ground water are based on the principle that if enough water is pumped from the site, the contaminants will eventually be flushed out. In such xe2x80x9cpump and treatxe2x80x9d systems, the pumped-out water is treated ex situ to remove contamination, which has limited effectiveness, especially for cleaning up undissolved sources of contamination beneath the water table. Key contaminant and subsurface properties that interfere with flushing include: solubility of contaminants into water; diffusion of contaminant into micropores and zones with limited water mobility; absorption of contaminants to subsurface materials; and heterogeneity of the subsurface. Because of the difficulty of flushing contaminants from the subsurface, the NRC concluded in its 1994 study that pump and treat systems would likely be unable to restore fully many types of contaminated sites.
During the 1990s, as the limitations of conventional subsurface remedial technologies have become increasingly clear, new technologies have emerged to clean up contaminated soil and leaking underground storage tanks containing petroleum products. Some of these newer technologies used on contaminated ground water at U.S. Superfund sites include air sparging, bioremediation, passive treatment wall, dual-phase extraction, in situ well aeration, in situ oxidation, and pump and treat. Those used to clean up contaminated ground water at underground storage tanks include biosparging, dual-phase extraction, air sparging, in situ bioremediation, pump and treat, and intrinsic remediation. Air sparging, dual-phase extraction, pump and treat, passive treatment wall, and in situ well aeration technologies include high equipment and labor costs with mechanical treatment of ground water. Bio- and intrinsic remediation have exhibited a long-term approach but are largely unproven, primarily owing to problems associated with providing an environment optimal for multiplication of the microbes while consuming the contaminant(s).
Systems have been known in the art for oxidizing hydrocarbons to harmless chemical constituents. A strong oxidizing agent known for such a use is hydrogen peroxide. In a reaction known as the Fenton reaction, hydrogen peroxide can be mixed with a metallic salt such as ferrous sulfate to produce a free radical, which breaks bonds in the hydrocarbon molecule in an exothermic reaction to produce a low-free-energy state, generally comprising a production of carbon dioxide and water.
Particular in situ systems utilizing Fenton-type reactions have been disclosed by Brown (U.S. Pat. N. 4,591,443) and Wilson (U.S. Pat. No. 5,611,642), both of which include mixing the Fenton reactants prior to introduction into the soil and ground water. Vigneri (U.S. Pat. Nos. 5,286,141 and 5,520,483) has described a remediation method and system that includes a preacidification of the ground water prior to a sequential introduction of the Fenton reactants, wherein hydrogen peroxide is added after an injection of ferrous sulfate at a high concentration.
It has also been found that some sites with certain geological characteristics consume remediation reactants; for example, high-carbonate soils, containing high concentrations of limestone or shells, can act as a buffer by raising the pH of the treating components and reduce their efficacy.
Further, some systems attempted to be used for remediation have been known to result in explosion owing to a lack of vapor extraction from an underground reaction site.
It is an object of the present invention to provide a system and method for remediating a site containing hydrocarbon contaminants.
It is also an object to provide such a system and method that are in situ.
It is another object to provide such a method and system for delivering a remediating reaction to a contaminated site.
It is a further object to provide such a method and system for controlling a remediating reaction within a contaminated site.
It is an additional object to provide such a method and system for providing an optimal pH at the contaminated site.
It is yet another object to provide such a method and system for extracting potentially harmful vapors from the contaminated site.
It is yet a further object to provide such a method and system for achieving penetration of the remediation effectors into a dense medium having a low value of hydraulic conductivity.
These and other objects are achieved by the method and system of the present invention, an in situ ground water and soil remediation treatment utilizing a Fenton-type reaction. It is to be understood here that the word soil is to be interpreted in its broadest sense, and should be taken to include components of ground as are known in the art, such as, but not limited to, various types of soils and clays.
In a first embodiment, the method comprises the steps of introducing a first compound into a site that contains ground water and soil contaminated with a hydrocarbon. Typically testing will have been undertaken to determine the location and extent of the xe2x80x9ccontaminant plume,xe2x80x9d that is, the spread of the contaminant throughout the site. The site is preferably essentially chemically untreated prior to the first compound""s introduction. The first compound is permitted to diffuse through the site, allowing sufficient time for the first compound to achieve equilibrium within the contaminant plume.
Next a second compound is introduced into the site. This second compound comprises an oxidizing agent that has the ability to react with the first compound to release a free radical in a Fenton-type reaction. The first and second compounds are permitted to react to release a free radical, which is permitted to act upon a site contaminant to form an environmentally neutral species. Exemplary first and second compounds comprise, but are not intended to be limited to, a metallic salt and hydrogen peroxide, respectively. As described above, the environmentally neutral species may comprise such compounds as carbon dioxide and water, although these species are not intended as limitations.
In a second embodiment of the invention, the first compound is introduced into a contaminated site and is permitted to diffuse therethrough, such diffusion aided by turbulence induced in the groundwater. The second compound is introduced, also with turbulence induced in the ground water to facilitate dispersion. Again, the Fenton-type reaction is permitted to occur, decontaminating the site.
In a third embodiment of the invention, a system is provided for remediating a contaminated site. In a particular embodiment, an injection portal is introduced into the site beneath the watertable, through which the first and second compounds can be introduced. Means are further provided for introducing turbulence into the ground water to enhance the diffusion of the compounds and their mixing. In a particular embodiment means are also provided for venting accumulated gases from the reaction site.
In all embodiments it has been found advantageous in certain geological systems also to provide a pH adjustment adapted to optimize the Fenton reaction. For example, as discussed above, high-carbonate sites can consume reactants by acting as a pH buffer and raising the pH of the groundwater upon an introduction of the reactants.
Advantageously, it has also been found that a particular turbulence-introducing means serves the dual purpose of providing a pH adjustment; namely, the introduction of carbon dioxide gas, which lowers the pH of the groundwater at the reaction site. It has also been found that the pH-adjustment means, e.g., CO2, can be added either before or during the reaction process; so the invention is not intended to be limited to the use of CO2 as the turbulence-creating means.
Further, an aspect of particular embodiments of the invention have been found to confer additional safety advantages. In particular, the vapors created and collected underground in some known systems must diffuse somewhere, typically along some xe2x80x9cpath of least resistancexe2x80x9d underground. Such diffusion has created serious hazards even remote from the cleanup site, leading to explosions. Thus an element of the present invention also prevents the occurrence of these hazardous conditions by extracting reaction vapors from the site before they can seep elsewhere.
Additionally, in prior art systems, such as those disclosed in the parent cases to the present application, reaction is predicated on sufficient soil porosity that the reactants can be transported to a desired reaction site. Therefore, in an alternate embodiment, means and steps are provided for enhancing reactant transport, wherein, prior to the first and the second introducing steps, the soil comprising the site is made porous, such as, for example, through physical means, including fluid fracturing methods such as hydraulic and/or pneumatic fracturing.
The features that characterize the invention, both as to organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description used in conjunction with the accompanying drawing. It is to be expressly understood that the drawing is for the purpose of illustration and description and is not intended as a definition of the limits of the invention. These and other objects attained, and advantages offered, by the present invention will become more fully apparent as the description that now follows is read in conjunction with the accompanying drawing.